1. Field of the Invention
This invention relates generally to the field of cable installation and specifically to an apparatus for feeding cable into a conduit.
2. Description of the Related Art
Cables are continually being installed to replace or supplement existing cables. For instance, fiber optic cables are installed to support growing communications networks. The fiber optic cables are installed in new locations or to replace electrical cables in existing locations.
The cables are commonly installed in passages, such as conduits or ducts, by applying an axial force to the cable. The axial force may be a tensile force applied at a leading part of the cable to pull the cable through the conduit. Alternatively, the axial force may be a compressive force applied to a trailing part of the cable to push the cable into the conduit. A preferred method utilizes both tensile and compressive forces.
Conduits or ducts are commonly plastic, metal, or concrete tubes defining an elongated passage in an aerial, submerged, underground, or other installation. A conduit may extend for several kilometers and includes curves and slopes. The interior surface of the conduit is preferably a low friction material or is provided with a low friction coating. The passage in the conduit should be free from obstructions to permit the cable to pass therethrough without interference.
Different apparatus and methods are known for applying the tensile and/or compressive axial forces on the cable for installation in the conduit. In some configurations, a fluid, such as air, is forced along the outer surface of the cable to create a drag force. The drag force tends to propel the cable through the conduit. In other configurations, the leading end of the cable is provided with a dart, also known as a pig, plug, drogue, etc. The dart is pushed by the fluid and pulls the cable. Some configurations are also provided with a driver, such as rotating wheels or belts, that engages the outer surface of the cable and pushes the cable into the conduit. Examples of these configurations and related components are shown in U.S. Pat. Nos. 4,756,510, 4,850,569, 4,934,662, 5,121,644, 5,156,376, 5,163,657, 5,197,715, 5,211,377, and 5,308,041, all incorporated herein by reference.
According to some prior art systems for installing cables, low pushing forces are utilized with high volumes of compressed air (350 cubic feet per minute or more). Such systems require large compressors to propel the cable through the conduit. Even with high volumes of air, inflexibility of the cable, obstructions, or curved conduit paths limit the effectiveness of the installation. In practice, these systems are limited to installing cables of about 2000 to 3500 feet in length and additional compressors are required along the installation path. Smaller compressors can be used when a more powerful pushing tractor is used. However, such tractors increase the risk of damaging the cable, which is designed to endure tensile and radial loads, not axial compressive loads. The resulting damage involves redistribution of the fibers, as well as excessive bending or kinking. In addition, the tractor belts are prone to slipping on the cable, thereby damaging the cable jacket.
It is desirable to maximize the length of cable that can be installed without interruption, and to reduce the size and number of air compressors required. As the length increases, the force required to continue installation increases. Fiber optic cables are often more delicate than metal cables. Therefore, the method and apparatus used for installation of fiber optics must not damage the cable while providing the necessary installation forces. In addition, the installation should be automatically controlled and use a minimum amount of equipment to improve efficiency.
These problems are largely solved in the present invention by using a lower air volume and a greater pushing force. Damage to the cable from the greater force is prevented by sensing the force applied to the cable. A feed back system controls the pushing force based on the force sensed. Pushing is also controlled based on the velocity of the cable. Damage is further prevented by limiting the radial force applied.
The number of compressors required is reduced by providing a "mid-assist" cable feeder with a by-pass pipe to convey pressurized air from one conduit or duct to a succeeding conduit or duct. With the present invention, distances exceeding 5,000 feet between cable feeders can be achieved and compressors are not required at mid-assist locations.